Industrial lubricating greases are homogeneous products of semi-liquid to solid consistency. Essentially, they consist of a dispersion of a thickener in a liquid lubricant or base oil. In general, the thickener is a significant determinant of the properties of the greases.
Polyurea compounds are among the thickeners used in making greases. Often the polyurea compounds are prepared directly in the base oil by the reaction of amines with isocyanates.
One method to test the performance of greases is cone penetration and prolonged working of lubricating greases through ASTM D217. A cone of specified weight is allowed to fall into a lubricating grease sample at 25° C. The depth of the cone, in tenths of a millimeter, identifies the consistency of the grease. With the use of Table 1, the NLGI grade of the grease is identified from the 60 stroke worked penetration.
TABLE 1NLGI Classification ScaleNLGI GradeASTM Worked Penetration000445-47500400-4300355-3851310-3402265-2953220-2504175-2055130-1606 85-115
This test can be used to determine the mechanical stability of a grease through prolonged working, such as 10,000 or more double strokes using the motorized grease worker. While cone penetrations are typically conducted at 25° C. measurements can be carried out at other temperatures. About 300 grams of grease are required to conduct the ASTM D217 test. ASTM method D1403, DIN 51 804, and IP 310 describe cone penetration equipment commonly referred to as ½ and ¼ scale devices for use when less than 300 grams of grease are available.
Dropping point of lubricating greases is used to determine high temperature structural grease properties related to the thickener. In ASTM D2265, dropping point of a lubricating grease is the temperature at which the thickener can no longer hold the base oil. Some of the reasons oil can no longer be held are that the thickener has melted or the oil has become so thin it is not held by thickener. Grease is placed in a small cup and heated in an oven-like device. When a drop of oil falls from the lower opening, the dropping point of the grease is calculated using the temperatures in the oven and inside the cup. Soap or polymer thickened greases demonstrate a dropping point while inorganic thickeners such as clay or graphite may not have a dropping point.
In U.S. Pat. No. 5,314,982 there is disclosed a process for making polyurea greases by first making a dry polyurea compound. Then the compound is pulverized to give powders having particles in the 100 to 400 micron range. Thereafter, a paste of the powder and base oil is heated, cooled and homogenized in a high pressure homogenizer at pressures of 400 to 1500 bar.
In U. S. Pat. No. 6,498,130 B2 a grease having low noise characteristics is made by shearing a base oil and thickener to reduce the thickener particles below about 500 microns in size. In this instance homogenization is achieved at about 2000 psi.
Both the above mentioned patents show the desirability of using polyurea of small particle size. They also illustrate that the conditions under which the polyurea particles are prepared impact subsequent processing conditions such as the homogenization step in grease forming.
User demand for polyurea greases has been increasing steadily; however, manufacturing such greases has been more difficult and expensive when compared to the manufacture of other greases.
One of the difficulties in manufacturing polyurea greases is consistently obtaining on an industrial scale substantially uniformly fine polyurea powder that is readily dispersible in a lubricating base oil.
Accordingly, one object of the present invention is to provide a process for making fine polyurea powder without the need for shearing or pulverizing large polyurea particles.
Another object of the invention is to provide a process for making fine powdered polyurea compounds which can be practiced on an industrial scale and that can be homogenized under standard grease homogenizing conditions.
Yet another object of the invention is to provide an improved method for preparing polyurea greases thus greatly reducing the risk (hazards) associated in manufacturing with neat amines or isocyanates.
These and other objects of the invention will become more apparent from the following description.